function findCommonStr(str1, str2){
    var low
    var high
    if(str1.length < str2.length){
        low = str1
        high = str2
    }  else {
        low = str2
        high = str1
    }
    for(var i = 0; i < low.length; i++){
        for(var j = 0, n = low.length - i; n != low.length + 1; j++,n++){
            var str = low.substring(j,n)
            if(high.indexOf(str) > -1){
                return str
            }
        }
    }
    return null;
}